Fine powder of polytetrafluoroethylene (hereinafter referred to as “PTFE”) is produced by coagulating polymer fine particles obtained by a so-called emulsion polymerization in which polymerization is effected in an aqueous medium with use of an emulsifier. It is technically known that PTFE is modified by copolymerizing tetrafluoroethylene (hereinafter referred to as “TFE”) with a relatively small amount of a comonomer copolymerizable therewith.
It is also known that modification of PTFE is effective for improving processability in paste extrusion of fine powder with an appropriate coagent as an additive.
Heretofore, such a method is known, as a polymerization method for a modified PTFE, that a modifier is added to a polymerization system before 70% of a predetermined amount of TFE is consumed. Known materials to be used as the above-mentioned modifier include a perfluoroalkyltrifluoroethylene represented by hexafluoropropylene (hereinafter referred to as “HFP”) and a chain transfer agent represented by methanol (JP-B37-4643).
Furthermore, it is conventionally proposed, as a polymerization method for a modified PTFE, to use two kinds of initiators consisting of a persulfate and a disuccinic acid peroxide in a system containing perfluoroalkyltrifluoroethylene or perfluoroalkyloxytrifluoroethylene as a comonomer (JP-B44-14937). It is also proposed to form a core-shell structure using a chlorotrifluoroethylene (hereinafter referred to as “CTFE”) comonomer (JP-B-56-26242).
Furthermore, it is proposed to add hexafluoropropylene (hereinafter referred to as “HFP”), ω-hydroperfluoroolefin or vinylidene fluoride after polymerization of 70% of the total amount of monomers (JP-B-56-26243). In addition, it is proposed to form a core-shell structure using HFP or ω-hydroperfluoroolefin as a comonomer (JP-B-57-18529).
It is known that the modified PTFE fine powder obtained by these methods is excellent in extrudability but low in heat resistance. Therefore, such a core-shell structure is proposed that a linear-fluoroalkyl or chain-fluoroalkyl vinylether is used for the core and CTFE is used for the shell, in order to improve the heat resistance (JP-B-04-3765). However, it is insufficient in terms of reliable heat resistance because it has thermally unstable CTFE.
Furthermore, another polymerization method is proposed for a modified PTFE, in which PTFE is modified using Rfa-CH═CH2 (Rfa is a perfluoroalkyl group) as a comonomer (JP-B-03-69926 and JP-B-08-26102). This method describes that the comonomer is continuously added up to a midway point of the polymerization in order to increase the degree of modification at an early stage.
However, this comonomer is also inadequate to achieve reliable heat resistance because its structure is not a perfluoro-structure but a hydrogen-containing structure.
In particular, further improvement in the heat resistance is recently required with increasing demands for the heat resistance of parts due to environmental measures to automobile exhaust emissions.
Furthermore, a polymerization method for a modified PTFE with improvement in the heat resistance is proposed, in which the heat resistance is improved by making a core-shell structure using a perfluorobutylethylene comonomer for the core and a HFP comonomer for the shell (at least 75% of the yield) (JP-A-09-87334). In addition, an improvement in the heat resistance is proposed by making a core-shell structure using a linear-fluoroalkyl or chain-fluoroalkyl vinylether comonomer for the core and charging a chain transfer agent for the shell (at least 80% of the yield)(the brochure of WO00/02935).
As described above, studies have been made on the use of the binary system of the persulfate and the disuccinic acid peroxide as a polymerization initiator, and on the TFE copolymers with both excellent paste extrudability and excellent heat resistance by the core-shell structure composed of the combinations of various comonomers conventionally known. However, the perfluoro monomer is generally low in reactivity and has a drawback in productivity and, for further improvement in the moldability, there have been needs for a comonomer having a novel structure.